FIELD OF THE INVENTION AND RELATED ART STATEMENT
The present invention relates to an electronic endoscope apparatus having a micro lens array on a photoelectric conversion surface of a solid state imaging device which converts a subject image into electric signals.
Recently, there has come into use of an electronic endoscope (which is also called electronic scope) which has a solid state imaging device such as a charge coupled device (CCD) within the tip part of the endoscope insertion part to take a picture image of a subject. Such an electronic endoscope is connected to a video processor through a connector and various kinds of picture processing are applied to video signals so that a picture image of the subject is displayed on a CRT monitor. On the other hand, by using a fiberscope which has been existent so far and connecting a television camera (TV camera) to an eyepiece part of the fiberscope, a type of electronic endoscope apparatus fitting a TV camera outside whereby a picture image of a subject is displayed on a CRT monitor is also used. The TV camera picks up an optical image of a subject transferred to the eyepiece part through an image guide fiber within the fiberscope.
The performance of an electronic endoscope which composes the electronic endoscope apparatus is required having an efficient solid state imaging device, especially having many pixels and being highly sensitive. However, for example, in a medical endoscope, a diameter of a tip part of an insertion part cannot be wider because the medical endoscope is inserted into a narrow tube cavity or the body cavity. Also, in an endoscope system wherein a TV camera is connected to an eyepiece part of a fiberscope, since an optical image is transferred to the eyepiece part through an image guide fiber, the quantity of light of an optical image which enters an imaging part of the TV camera is attenuated or sensitivity is lowered or interference moire is generated by a pitch of a fiber and a pitch of a pixel of a solid state imaging device so that it has been necessary to insert an optical filter in an observing optical path.
As mentioned above, the solid state imaging device provided within the tip part of the endoscope insertion part is assigned a requirement of highly efficient pixel and smaller size which are contrary to each other. In order to make the solid state imaging device smaller while its high sensitivity is maintained, it is considered that a line transfer type CCD wherein a sensor part (light sensitive part) 101 and a vertical transfer part (V.CCD, hereinafter) 102 are combined as shown in FIG. 1 is the most suitable one. Also, a horizontal transfer resistor 103 is provided on one end of the sensor part 101 (and V.CCD 102). In this line transfer type CCD, because a smear ring is generated when an incident light enters at the charge transferring time, it is necessary to shield the light to prevent the smear ring.
Where the line transfer type CCD is used in the electronic endoscope apparatus, an exposure period and a shielding light period are alternately provided as shown in FIG. 2 so as to store a charge during the exposure period and transfer the charge and read it during the shielding light period.
Also, recently, a measure, that is, a treatment under an endoscope by laser beams or the like has been required for an endoscope examination. Both of the laser beams (invisible light) and visible lights are continuous, therefore, in an electronic endoscope apparatus using a line transfer type CCD, severe blooming phenomenon is caused by incident laser beams or visible guiding lights during a charge transferring period of the CCD so that the quality of a picture has been badly damaged and that a fine treatment in details has been affected. FIG. 3 shows an example of an endoscope picture image during the laser beams application in the electronic endoscope apparatus using the line transfer type CCD. As shown in FIG. 3, if laser beams 106 emanated from a laser probe 105 is applied to an affected part 104, a blooming 107 is generated in a neighboring part of the part applied by the laser beams 106.
Then, it is considered to use a system in which shielding is not needed, for example, a frame storage type (it is also called a frame transfer type) CCD or an interline type CCD. In the frame storage type CCD, a sensor part 101 and a storing part 108 are separately arranged as shown in FIG. 4 and a horizontal transmission resistor 103 is provided on one end of the storing part 108, which is the opposite side of the sensor part 101. On the other hand, in the interline type CCD, the sensor part 101 is arranged being adjacent to the V. CCD 102 as shown in FIG. 5.
In the frame storage type CCD, the storing part 108 is needed in addition to the sensor part 101 as mentioned above and there is a problem in which a tip part size becomes larger when an electronic endoscope having a solid state imaging device at the tip of the insertion part is used. Also, in the interline type CCD, if it is compared with the line transfer type or the frame storage type by a pixel of the same size, the size of the sensor part 101 is reduced by about half because a sensor device (sensitivity device) part 101 and a pixel part of a V. CCD part 102 which is adjacent to the sensor device part 101 compose one pixel as shown in FIG. 5 so that there is a problem in which an aperture ratio is lowered and the sensitivity is reduced by half.
An outside fitting electronic endoscope in which a TV camera 120 fitted outside is connected to an eyepiece part 113 of a fiberscope 110 is constructed, for example, as shown in FIG. That is, the fiberscope 110 comprises an insertion part 111, an operating part 112 fitted to the rear end of this insertion part 111, the eyepiece part 113 provided at the rear end of this operating part and a light guide cable 14 extended from the operating part 112. A luminous intensity distributing lens and an objective lens 116 are provided at the tip part of the insertion part 111. A light guide fiber 117 is fitted to the rear end of the luminous intensity distributing lens 115. This light guide fiber 117 is inserted into the insertion part 111, the operating part 112 and the light guide cable 114 and an incident end part is connected to a light source apparatus 130. This light source apparatus 130 comprises a lamp 131, a power source 132 supplying power to this lamp 131 and a lens 133 converging light emanated from the lamp 131 and making the light enter the incident end of the light guide fiber 117. Also, a tip surface of an image guide fiber 118 is placed in an image forming position of the objective lens 116. This image guide fiber 118 is inserted into the insertion part l and the operating part 112 and the rear end surface of the image guide fiber 118 is opposed to an eyepiece lens 119 within the eyepiece part 113.
The TV camera 20 comprises an image forming optical system 121 which forms an image observed by the eyepiece part 113, a CCD 122 arranged in the image forming position of this image forming optical system 121 and an optical crystal low pass filter (LPF, hereinafter) 123 arranged on the observing optical path which leads to this CCD 122. The TV camera 120 is connected to a video processor 140 through a cable 124. This video processor 140 comprises a driving circuit 141 connected to the CCD 122 through a signal line inserted into the cable 124 and a video signal processing circuit 142. The CCD 122 is driven by the driving circuit 141 and an output signal of the CCD 122 is processed at the video signal processing circuit 142. Then, a video signal from the video signal processing circuit 142 is supplied to a CRT monitor 145 on which a subject image is displayed.
This outside fitting electronic endoscope has a problem in which moire stripes (false signals) are generated because of a spatial sampling of the CCD 122 for a pitch of each fiber of the image guide fiber 118. Also, if the interline type CCD is used as the CCD 122, the sizes of an aperture part (sensor part 101) and a shielding light part (V.CCD 102) fitted to the aperture part become a pitch P of an unit pixel of the spatial sampling as shown in FIG. 7. As a counterplan for the moire, the LPF 123 is inserted in the observing path so as to reduce the moire phenomenon under existing circumstances. As shown in FIG. 8, each fiber 151 of the image guide fiber 118 is arranged, and for example, all fibers are symmetrically arranged about the dark shading fiber in this diagram. Up to the present, sampling turning back distortion has been removed by a crystal filter as the LPF in the spatial direction of at least (1)-(4) in FIG. 8 in order to remove the moire. Also, in FIG. 8, P1 is a pitch of the image guide fiber 118 in the direction (1). In this case, the number of crystal filters which is the same number of the direction desired to be removed is needed and it amounts to four to six. The necessary thickness of each crystal filter t(mm) is 0.17.times.P1 in thickness for the pitch of the fiber P (mm) in the direction desired to be removed. As shown in FIG. 9, the LPF 123 consists of six filters has more than 10 mm in thickness and it was a big defect in its size, weight and price. Especially, in the TV camera 120 fitted outside of an endoscope, since a doctor keeps holding the TV camera during the medical examination and treatment, the increase in size and weight of the TV camera was a big weak point. Further, the high price of the six filters' composition makes the product costs higher.
Another method for removing the moire is to use a soft focusing lens utilizing spherical aberration as stated in Japanese Patent application No. 115107 of 1990 which was applied by the assignee. In this case, as shown in FIG. 10, at the same time that a modulation transfer function MTF falls as shown by the broken line, the threshold resolution also falls as compared with the case in which a soft focusing lens is not used as shown by the solid line. Also, FIG. 10 shows the MTF attached the condition of the pitch P of the unit pixel in the sampling in FIG. 7.